This invention relates to an electrolyte for an electrolytic capacitor and an electrolytic capacitor using the same.
An electrolytic capacitor uses a so-called valve metal anode such as as aluminum and tantalum, on which an insulating oxide film is formed by anodic oxidation as a dielectric material. The electrolytic capacitor can be formed by placing a cathode opposed to the anode, interposing a separator between two electrodes and making retained an electrolyte to the separator.
Since the anode is subjected to etching treatment to enhance the surface area, the electrolyte functions as a true cathode by contacting with the concave and convex surface. Electric conductivity and temperature characteristics of the electrolyte become main factors to determine the electric characteristics of an electrolytic capacitor. Also, the electrolyte repairs damage of an oxide film whereby it affects to leakage current and lifetime of the capacitor. Thus, the electrolyte is the most important constitutional element which controls the characteristics of the electrolytic capacitor.
In the prior art, as an electrolyte for an electrolytic capacitor, particularly for a medium or high voltage, so-called ethylene glycol-boric acid type electrolyte has been used. This kind of the electrolyte forms water when heating by esterification reaction of ethylene glycol and boric acid. Therefore, there is a disadvantage that it cannot be used at a high temperature exceeding 100.degree. C. since inner pressure is too heightened.
In order to overcome such a disadvantage, azelaic acid, sebacic acid, decanedicarboxylic acid or a salt of these acids has been used, but these materials are poor in solubility to a solvent such as ethylene glycol and also insufficient in thermal stability.
In Japanese Laid-Open Patent Application No. 116815/1986, there has been proposed the method for improving solubility and thermal stability by using a tertiary monocarboxylic acid, but anodic film forming ability is insufficient. In Japanese Laid-Open Patent Application No. 103821/1989, it has been proposed to use a tertiary dicarboxylic acid, but the compound contains ester bonds, which are subjected to scission, and results in the deterioration of the electrolyte.